1. Field of the Invention
This invention relates to a data storing unit for a camera which operates to store film count data and data necessary for operating the camera.
2. Description of the Background Art
A microcomputer is generally used to control a variety of operations of a camera. In this connection, it is necessary to use a memory for storage and renewal of the various data required for control of the microcomputer. The memory is generally a RAM (random access memory) integral with the microcomputer. The RAM is volatile and therefore lose its contents when the power supply is cut off. Accordingly a non-volatile memory which does not lose its contents even if the power supply is turned off (hereinafter referred to as "an E.sup.2 PROM," when applicable) is employed together with the microcomputer. Film count data generated as corresponding to every film winding operation and various data necessary for the operations of the camera, such as data representing the shutter's exposure operation and data indicating the fact that the film is being wound or rewound, are stored within the non-volatile memory.
The E.sup.2 PROM retains its contents as they are even when the power is cut off. Therefore, in general, no backup power source (capacitor) is provided for the E.sup.2 PROM. If, however, while data are being written in the E.sup.2 PROM, the power supply is cut off, for instance by removal of the battery, data can no longer be written into the E.sup.2 PROM and the data stored therein may be lost.
A first method of overcoming the above-noted difficulties is set forth in U.S. Pat. No. 4,733,265 and is disclosed as follows: A switch is provided in the battery chamber which detects intended removal of the battery before it is actually removed, so that abrupt cut off of the power is prevented whereby power supply time necessary for writing data in the E.sup.2 PROM is secured. In addition, a second method is disclosed in Japanese Patent Application (OPI) No. 61731/1985 (the term "OPI" as used herein means an "unexamined published application") in which abnormal battery voltage is detected and the contents of the RAM are thereafter written into E.sup.2 PROM before the power is cut off.
In the above-described first prior art method, the film count data and the data necessary for the operations of the camera are stored in the RAM in the microcomputer, and the switch detects cut off of power in advance so that the data are transferred from the RAM into the E.sup.2 PROM. The detecting switch operates much earlier than actual cut off of the power; for instance, in operates at the start of the removal of the battery. Thus, a power supply time period long enough to ensure writing of the data in the E.sup.2 PROM can be obtained.
However, the first prior art method is disadvantageous in that, since it is necessary to provide a mechanism such as a switch for detecting the removal of the battery from the battery chamber prior to the actual removal of the battery, it is necessary to provide space for the mechanism which results in increased manufacturing cost.
In the second prior art method, upon detection of the abnormal voltage of the battery, the data are written in the E.sup.2 PROM. However, it is considered that, at the instant of detection, it is rather difficult to obtain a power supply time period long enough to write the data. In addition, with respect to the second prior art method, in order to eliminate the use of the backup power source, in a manner similar to the first prior art method, it is necessary to provide a mechanism for detecting cut off of the power before such cut off actually occurs.
Therefore, as described above with respect to the prior art, in order to prevent cut off of power during the period in which being written in the E.sup.2 PROM, it is necessary to provide means for detecting cut off of power prior to actual cut off or to provide a backup power source. As a result, the camera is accordingly more intricate in construction and of increased manufacturing cost.